Comparison of a wearable core temperature sensor with rectal thermometry during high-intensity interval running in pregnant and non-pregnant women

Background: /

Objective: An increasing number of pregnant women, including elite athletes, continue vigorous- and high-intensity exercise during pregnancy. Current pregnancy exercise guidelines primarily recommend at least 150 minutes of moderate-intensity physical activity per week and provide limited specific guidance on vigorous- and high-intensity exercise in highly trained pregnant women. High-intensity exercise can elevate core temperature (Tcore), raising concerns about maternal hyperthermia and potential fetal risks (e.g. neural tube defects, preterm birth, fetal growth restriction). Accurate monitoring of maternal Tcore may be necessary to evaluate safety. However, pregnancy-related cardiovascular and thermoregulatory adaptations, such as changes in skin blood flow and heat dissipation, may affect temperature estimates, yet few non-invasive and user-friendly

Methods: combine accuracy with practicality during exercise. This study compared the non-invasive CALERA Research Sensor (CRS) with a rectal probe, the gold standard for Tcore measurement, during high-intensity interval running in pregnant and non-pregnant participants.

Methods Thirty elite and recreational athletes participated, including 15 pregnant participants (31 ± 4 years) and 15 non-pregnant participants (30 ± 6 years). All completed a separate 10-minute warm-up, followed by up to five 5-minute high-intensity treadmill-intervals separated by recovery periods. Testing was conducted under environmental conditions of 20-21˚C and 38-61% relative humidity. Analyses included the intervals, corresponding pauses, and 20 minutes of post-exercise, during which no cool-down was performed. Agreement was assessed using Bland–Altman plots, with a pre-defined ±0.3°C acceptability threshold. Differences were analyzed using paired t-test.

Results: Nearly half of the measurements exceeded the predefined threshold (47% in pregnant vs. 43% in non-pregnant participants), reflecting systematic bias between

methods. In pregnant participants, the mean difference (rectal probe - CRS) was -0.23°C (95% CI: -0.36, -0.09°C), indicating that CRS overestimated Tcore relative to the rectal probe. In contrast, in non-pregnant participants, the mean difference was 0.16°C (95% CI: 0.03, 0.28°C), indicating that CRS underestimated Tcore. Discrepancies were most pronounced during the early intervals but diminished over time.

Conclusion: This study shows opposing systematic bias of CRS in pregnant and non-pregnant women compared with Tcore measured using a rectal probe, limiting its use as a stand-alone tool in pregnancy and highlighting the need for non-invasive temperature sensor development.